Thanks for continuing the discussion, which I hope is useful -- certainly for me.
I note that others have noted that the EGLL SIDs climb to 6000' and depending on the pressure FL70 may not be separated.
Indeed this was one of those days last week where 6000 ' and FL70 were virtually coincident (QNH 987). We didn't discuss if FL80 was available eastbound as FL90 turned out fine on the day.
Actually it's not the reason for having IFPS in the first place, it's a side benefit.
But it
is the reason for having FPLs, airways, MEA and the RAD. It's important that operators have a good idea before flight of the routes that are and are not acceptable to ATC. Otherwise, we may end up making assumptions that get us into trouble. It's a great deal easier to know that FL70 is not available in advance on the ground rather than in the climb in potentially icing conditions having already selected the fuel load. There will always be the need for flexibility, but the closer we can get a correspondence between flight planning and real routings and levels the safer it is. Which is one of my motivations for asking what the practical limits are.
Unfortunately (and I speak as one of them) I doubt anyone would consider the TMA min overflying level before authorising departure/issuing clearance. In fact the immediate information is not to hand on sector, unless its self-calculated.
Strange then. I can't see how Lyneham traffic would have been relevant to the choice of level. Bristol cleared me to join in the climb to FL90 on track MALBY (and with a 50 knot westerly I'm not sure I made 90 by MALBY) and implied that the unavailability of FL70 was at London's request.